The Collaborative International Dictionary of English v.0.48:

Landgrave \Land"grave`\, n. [G. landgraf; land land + graf earl,
   count; cf. D. landgraaf, F. landgrave.]
   A German nobleman of a rank corresponding to that of an earl
   in England and of a count in France.
   [1913 Webster]

   Note: The title was first adopted by some German counts in
         the twelfth century, to distinguish themselves from the
         inferior counts under their jurisdiction. Three of them
         were princes of the empire.
         [1913 Webster]

WordNet (r) 3.0 (2006):

landgrave
    n 1: a count who had jurisdiction over a large territory in
         medieval Germany